As fellow blogger Dalmasian noted in his comment 6, on "More Evil in Tibet",  whilst agreeing that it is a good thing that we can freely discuss such things, "many people who supposedly witnessed sightings of UFOs are afraid of being ridiculed in public and have elected to keep their mouth shut."
There is a very (very) simple reason for this: much of the speculation about UFOs is so profoundly idiotic that no sensible person would wish to be sullied by association.

First, are there UFOs? Yes. No sensible person would argue otherwise.
What does this mean? Well, it means that there are object that fly around and for which fairly certain identification is not possible. That is, we don't know what they are. Sometimes when I'm walking through Lumpini Park, something whizzes through my field of vision, and I don't know it it's a bug or a bit of blown leaf. That is a UFO. Sometimes people have a perception of something in the sky, and they don't know what it is. That's a UFO. Sometimes pilots have them, just like I do. Sometimes something is captured on film that can not be readily identified: another UFO.

Now comes the interesting bit.
Some people feel, on the basis of precisely zero, as in none whatsoever, evidence that some or all UFOs must be alien spacecraft visiting our planet. There is not now and never has been a shred of verifiable evidence to support such a belief. And the extreme improbability of such an explanation renders it literally incredible. There might be some such evidence next week, which will be truly exciting. There may well be highly intelligent life elsewhere in teh universe, but to date, we have zero evidence to say that there is.

This presents a real problem for those with a compulsion to entertain such a belief about UFOs, so they create the fiction of a government cover up. There is, of course, no supporting evidence for this fiction. Worse, the fiction usually involves all major world governments, over a period of decades. This, sadly for the UFOologists, creates an even bigger problem: How to explain such perfect and sustained cooperation between states that are normally at loggerheads over everything, some of whose governments change abruptly to oponents of the former rulers. THis is so deeply contrary to the readily observable facts of human nature that it requires the extra belief that all senior politicans are actually aliens or fully remote controlled by aliens, who maintain a facade of human behaviour with the typical conflicts and all the rest thrown in. At this point, the circle has to widen a little more, because any close aids would notice the disccrepancies, so the widening circle eventualy reaches the point where the only person on earth who isn't actually an alien or in the employ of aliens from outer space is the devout UFOologist.

The semi-rational believer then asks himself: But why would the aliens go to all that trouble just to delude me? Why am I that special? Why replace everyone else on earth and pretend to cover up when I'm actually the only person on earth who is still a person? Wouldn't it be easier just to take me home with them?
We leave these ultimate questions for the solipsistically inclined UFOologist to answer. They take us into unchartered psycholgical territory where there be unicorns and other Unidentified Fantasy Objects, along with crystal skulls imbued with who knows what esoteric knowledge, pyramid homes on Mars where live ancestral humans of terrible power, and so on.